Sayyiduna Awf bin Malik (may Allah be pleased with him) came to Ka'b while he was preaching, and said: I heard the Prophet (peace and blessings be upon him) say: "No one should preach except a ruler, or one appointed by him, or one who is being pretentious." So Ka'b stopped preaching until Mu'awiyah (may Allah be pleased with him) ordered him to do it.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15231